1. Field of the Invention
This invention relates to a record rewritable optical disk apparatus of the phase difference type, and more particularly to an erasing optical head for use with a record rewritable phase difference optical disk apparatus and a recording and reproduction method by an optical equivalence method.
An optical disk apparatus is a storage apparatus of the contactless type which records information in a high density onto an optical disk and reproduces the information at a high speed using a laser beam, and is being investigated and developed as an external storage apparatus for an information processing system which has both of a high speed accessing property of a magnetic disk apparatus and a large storage capacity of an optical disk. It is forecasted that an optical disk apparatus which has high transfer rate and large memory capacity will be used for all disk storage apparatus for use with a high definition television (HDTV) system of the next generation.
Some optical disk apparatus are constructed only for reproduction of a record stored an optical disk, and some other optical disk apparatus are constructed so that they can rewrite a record of an optical disk. The present invention principally relates to an optical disk apparatus of the phase difference type which uses a record rewritable phase change medium.
2. Description of the Related Art
An optical disk for the phase difference type is constructed so as to provide a phase difference between the phase for reflected light from a "mark" portion of information and the phase for reflected light from a "blank" portion of the information. As shown in a partial sectional view of FIG. 1, an optical disk of the phase difference type includes a base plate 11 in the form of a transparent disk of polycarbonate, a first protective film 12 in the form for a composite film (ZnS--SiO2) of zinc sulfide and silicon dioxide, a recording film for a phase change medium 13 of a three element type with germanium-antimony-tellurium (GeSbTe) or the like, a second protective film 14 of Zn--SiO2, and a reflection film 15 of aluminum (Al) or silicon (Si). The first protective film 12, the phase change medium 13, the second protective film 14 and the reflection film 15 are formed in this order on the base plate 11. Writing or erasure for information onto or from a rewritable optical disk is performed by changing the condition of the phase change medium 13 between a crystal condition and a non-crystal condition, and a portion in a non-crystal condition is recorded as a mark 16. The phase difference between reflected waves from a mark and a blank is normally set to approximately 180 degrees so as to assure a high resolution.
Light irradiated from an objective lens 75 is introduced into the optical disk from the base plate 11 side, passes through the first protective film 12, the phase change medium 13 and the second protective film 14 and is reflected by the reflection film 15. The reflected light reversely follows the same route and is introduced into the objective lens 75 again. The incident light to the objective lens 75 exhibits a phase difference between a component thereof which has passed through a portion in a non-crystal condition of the phase change medium 13 representing a mark and another component thereof which has passed through another portion in a crystal condition of the phase change medium 13 which represents any other than a mark.
An optical equivalence method is one of data reproduction methods for an optical disk disclosed, for example, in Japanese Patent Laid-Open Application No. 60-23932 (1985) or No. 63-58625 (1988). According to the optical equivalence method, an optical disk on which information is recorded is scanned by a laser beam irradiated from an optical head, and reflected light from the optical disk is detected by an optical sensor which is divided at least into two portions of a front half portion and a rear half portion in the scanning direction. The output of the front half portion of the optical sensor is delayed and added to the output of the rear half portion of the optical sensor to reproduce a signal corresponding to the information recorded on the optical disk.
An example of a construction typical of the conventional optical heads used for the optical equivalence method is shown in FIG. 2. Referring to FIG. 2, the optical head shown includes a laser light source 71 of a wavelength .lambda., a collimator lens 72, a polarizing beam splitter 73, .lambda./4 plate 74 of the quarter wavelength, an objective lens 75, a convergent lens 76, an optical sensor 77 having four divisional portions or at least two divisional portions, a pair of amplifiers 78 and 79, a delay unit 80, and an adder 81. It is to be noted that, although an actual apparatus includes, in addition to the components mentioned above, a circuit for rotational control of an optical disk, a servo circuit for positioning the optical head and other necessary circuits, they are omitted in FIG. 2.
Light from the laser light source 71 is converted into parallel light by the collimator lens 72, passes through the polarizing beam splitter 73 and is converted into circularly polarized light by the .lambda./4 plate 74. Then, the circularly polarized light is converged by the objective lens 75 and irradiated as a convergent beam spot upon an optical disk 82. Reflected light from the optical disk 82 is introduced through the objective lens 75 and the .lambda./4 plate 74 into the polarizing beam splitter 73, by which it is deflected by 90 degrees. The light from the polarizing beam splitter 73 is converged by the convergent lens 76 and introduced into the optical sensor 77.
The optical sensor 77 has at least two divisional optical detection portions, which correspond to a front half F and a rear half R of the convergent beam spot, which advances as the optical disk 82 rotates, in the advancing direction of the convergent beam spot. As the optical disk 82 rotates, that is, as the convergent beam spot advances, the optical sensor 77 outputs detected signals from the front and rear detection portions F and R thereof. The two output signals are amplified by the amplifiers 78 and 79, respectively, and after the output signal of the amplifier 78 is delayed by the delay unit 80 in accordance with the necessity, the two signals are inputted to and added by the adder 81. An RF signal (Radio Frequency signal) for the addition is outputted as a reproduction signal of the stored information of the optical disk 82.
When a reproduction RF signal is obtained by the apparatus described above, the correspondence between the relative positional relationship between a light beam spot on the surface of a medium and a pit representing a mark and a diffraction image at an aperture portion of the objective lens 75 or on a light reception surface for the optical sensor 77 is such as shown in FIG. 3. In particular, when a leading end of a mark 32 advances into a light beam spot 31 as seen in FIG. 3(a) and when only a trailing end of the mark 32 remains in the light beam spot 31 as seen in FIG. 3(c), a small image is produced at a central portion of the optical sensor 77 as seen in (d) and (f). But when the center of the light beam spot 31 and the center of the mark 32 are aligned with each other as seen in FIG. 3(b), a diffraction image is produced over the entire area of the optical sensor 77 as seen in FIG. 3(e). As can be seen apparently from FIG. 3, at whichever position the mark 32 is in the light beam spot 31, the diffraction image of the mark 32 is positioned at a central portion of the light reception sensor. In short, the outputs of the light reception sensor are quite the same between the front half side and the rear half side, and the outputs of the amplifiers 78 and 79 are such as shown in FIG. 4.
An optical disk apparatus which can record, reproduce and erase information includes, as shown in FIG. 5 three optical heads including an erasing head 102, a recording and reproducing head 103 and a reproducing head 104 which are disposed on a track of a same circumference along a direction 101 of rotation of an optical disk 100. The optical heads 102, 103 and 104 irradiate single laser light from a semiconductor laser as a beam upon a track of the optical disk, read an address written on the optical disk in an applied condition of tracking servo and erasure, writing and reproduction of a record for the same track.
In order to realize an optical disk apparatus of a high transfer rate and a large memory capacity, it is required to minimize the diameter of the convergent beam and raise the erasure rate of the erasable optical disk. Further, where the optical disk is an optical disk of a phase change medium or the like onto and from which information can be recorded, reproduced and erased, the erasure rate must be set high in order to achieve a high carrier to noise ratio (CNR).
The CNR of a reproduction signal can be raised by increasing the contrast based on the difference in reflection factor between a crystal condition and a non-crystal condition (amorphous) of a phase change medium, that is, by increasing the optical-medium modulation degree. Further, in the optical disk of the phase change type, the optimum erasing power is intermediate between the optimum recording power and the optimum reproduction power.
An erasure occurrence number to CNR characteristic of the phase change optical disk described above at a high speed is illustrated in FIG. 6. Referring to FIG. 6, the axis of abscissa indicates the erasure occurrence number, and the axis of ordinate indicates the CNR characteristic. From FIG. 6, it can be seen that a single erasing operation cannot sufficiently erase a record and two or more erasing operations are required to erase a record into a substantially saturated condition.
With the phase change optical disk, since the CNR characteristic by recording and reproduction cannot be extracted sufficiently by a single erasing operation by which sufficient erasure cannot be achieved as described above, a total storage modulation degree by an optical disk and an optical head which is most important to achieve high density recording, that is, the product between the medium modulation degree and the optical modulation degree, cannot be increased sufficiently. Accordingly, high density recording cannot be realized.